In analog and digital transmission systems the transmitted voice, sound, data etc. has to be terminated to different impedances. For example, in an analog trunk line unit the input impedance has to be selected e.g. between a resistive and complex impedances. From these the resistive impedance is an ohmic resistance and the complex impedance is a more complicated combination of ohmic resistances and capacitances. Sometimes even inductances form a part of the latter. The numeric values thereof might be such as e.g. 600 .OMEGA.(res.) or 220 .OMEGA.+(820 .OMEGA.//115 nF) (complex), respectively.
The known designs have used straps, mechanical switches or mechanical relays for setting i.e. matching the suitable impedance. Such means need to be turned (switched) on and off manually e.g. by a service man. If the set impedance level is not in the predetermined level an echo effect will exist. In addition, there is always a risk that such mechanical means are switched to a wrong position. This might even happen accidentally after the initial, per se correct settings. From these known means the relays might be software controllable, but the power dissipation, size and lifetime thereof has not proven to be at a acceptable level. The use of so called optorelays is not adequate, as optorelays of good quality are expensive and as the on-state-resistance thereof will affect the termination impedance.
When selecting impedance said on-state-resistance should be zero. On-state-resistance means a unidealality of a switch, i.e. the value of the ohmic resistance over the switch as the switch or relay "draws" i.e. switches. In mechanical relays said value is almost zero, but such relays have various other disadvantages, such as the size, the slow switching speed and overheating problems. Therefore other solutions have been sought, and in many applications said resistance has been accepted to avoid the use of the mechanical relays.
The term `terminating point` is usually used for a point in which the termination impedance can be seen against the ground. The value of the termination impedance in the terminating point may vary according to the overall system used and the national and international recommendations or requirements in various market areas and countries. For example, a similar termination impedance is not in use in Germany and in USA. In the following the term `market`is intented to generally mean one such country, market area or similar having an impedance of its own.
For the better understanding of the invention, a brief explanation of the directions in which a signal may move in a terminating point of a signal path is also given herein for three alternative events, i.e. for 1) terminating impedance, 2) output impedance, and for 3) input impedance. In the event of the terminating impedance 1), the signal moves both in and out in the terminating point. This kind of use is common for example in telephone equipments in which the users may listen and speak simultaneously, in fax machines, modems, trunk line units etc. When used as an output impedance 2), the signal moves only out from the terminating point. This kind of use is common, for instance, in generators or measuring devices which generate an output signal (e.g. 1014 Hz, 245 mV) at a given impedance level (e.g. 600 .OMEGA.). The third possibility, i.e. input impedance 3), is commonly used e.g. in level meters of measuring devices metering the voltage level of an input signal (power level, disturbance level etc.). In this case the signal moves only to the `in` direction at the terminating point.